1. Field of the Invention
The present invention relates to a display such as a liquid crystal display, and more particularly to a diffusing sheet for use in a surface light source unit that illuminates a transmission type liquid crystal display or the like from its rear, to a surface light source unit, and to a transmission type display using the diffusing sheet and the surface light source unit.
2. Background Art
A variety of surface light source units that are used to illuminate transmission type liquid crystal displays or the like from their rear have been proposed and put into practical use. Such surface light source units are broadly classified into the edge light type and the direct type according to the mode of conversion of a non-surface light source to a surface light source.
Of these, a surface light source unit of direct type is so constructed that light is introduced to the rear of a transmission type display member serving as a light bulb, such as an LCD panel, directly from cathode ray tubes arranged in parallel. In such a surface light source unit of direct type, the cathode ray tubes and the transmission type display member are properly spaced, and in this space are placed a diffusing sheet and a combination of two or more optical sheets capable of converging light.
Such a conventional surface light source unit of direct type is poor in the light-converging properties although the number of optical sheets needed for this unit is great. In order to solve this problem, the structure of the transmission type display member itself, such as an LCD panel, has been improved so that even light that is obliquely incident on the display can produce an image of excellent qualities. However, this conventional way of improvement causes reduction in optical efficiency, and also makes the transmission type display member such as an LCD panel complex in construction, which leads to increase in cost.
Another disadvantage of the conventional surface light source unit of direct type has been that the light intensity (luminance) on the display member tends to be non-uniform depending on the distance from the cathode ray tubes (i.e., whether a certain point on the display member is close to the cathode ray tube or to the space between the cathode ray tubes that are arranged in parallel).
A possible method of avoiding the above-described non-uniformity is to well space the cathode ray tubes and the transmission type display member such as an LCD panel. This method has been disadvantageous in that the display has an increased total thickness.
Another possible method of avoiding the above-described non-uniformity is to increase the degree to which the optical sheets or the like that are placed between the cathode ray tubes and the transmission type display member such as an LCD panel diffuse light, or to control the amount of light which the optical sheets transmit. This method has been disadvantageous in that it causes decrease in the amount of usable light.
More specifically, Japanese Laid-Open Patent Publications No. 119703/1993 and No. 242219/1999, for example, propose a method for maintaining the uniformity of light, in which a light-shielding member such as a lighting curtain or a light-shielding dot layer is provided in a surface light source unit. This method has been disadvantageous in that it causes decrease in the amount of usable light, like the above-described method.
Further, Japanese Laid-Open Patent Publication No. 347613/1994 proposes a method in which, in a surface light source unit, the diffusion of light in two directions is controlled by a sheet having lenticular lenses on both surfaces. Since the function of converging light cannot be sufficiently obtained by this method, the optical axis on every area on the face of the transmission type display member such as an LCD panel changes according to the position, relative to the cathode ray tubes, of the area. Thus, this method still has been at a disadvantage in that the brightness of light on the display screen varies depending on the position from which the display screen is observed.